HandsOn 16 - The Hele-Shaw Experiment with Carrageenan

I. Introduction

II. Preparing Carrageenan

III. Observational Analysis

II. Preparing Carrageenan

Carrageenan is available from chemical supply companies (e.g., Sigma). Here, we describe how to prepare carrageenan solutions of various concentrations and pH. You will need:

• 2.5 g Carrageenan
• 100 ml Distilled Water
• 2 ml Hydrochloric Acid (HCl)
• Two 250 ml glass beakers
• pipette or dropper
• pH paper for the range pH 3 through pH 5, or pH meter (see below)
• parafilm

1. Fixing the concentration of carrageenan solution. Carrageenan comes as a powder, and is slow to dissolve in water. Be prepared to let the solution stand for at least 24 hours to permit the powder to dissolve. The resulting solution is also an excellent medium for the growth of bacteria and mold, and will become contaminated. Hence, it should be used within a few days of preparation. To begin try a concentration of 25 mg/ml. Add 2.5 g of carrageenan to a quantity of water less than 100 ml of distilled water. Then add enough water to complete a volume of 100 ml. Cover the solution (e.g., with parafilm) and leave it overnight for the powder to dissolve. After the carrageenan has dissolved, mix the solution for a few seconds to make it homogeneous. Other concentrations may be made following these same guidelines but varying the amount of carrageenan per 100 ml distilled water.


2. Fixing the pH of the carrageenan solution. We recommend practicing the following procedure on a small quantity of your solution first. Initially the carrageenan solution will have a pH of between 8 and 9. Prepare a dilute solution of HCl acid (e.g., 2 ml of HCl (36.5-38%) in 100 ml of distilled water). Add the diluted HCl dropwise to your carrageenan solution, and mix the solution. Measure the pH of your solution using a pH meter (directions below) or with pH paper. By repeating this process, prepare carrageenan solutions of pH 3 and pH 5.


3. To adjust the pH of "pure'' water, add 1 ml of HCl to 100 ml of distilled water then take its pH. Adjust the pH by adding distilled water.

Once you have the desired pH, add food coloring to the HCl solution. This will provide contrast between the HCl solution and the carrageenan when performing the Hele-Shaw experiment.

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